1. Field of the Invention
The present invention relates to a method and apparatus for detecting and correcting an unbalanced condition in the rotating drum of a washing machine. It is particularly applicable to a washing machine having a drum on an axis other than vertical.
2. Description of the Related Art
Washing machines utilize a generally cylindrical perforated basket for holding clothing and other articles to be washed that is rotatably mounted within an imperforate tub mounted for containing the wash liquid, which generally comprises water, detergent or soap, and perhaps other constituents. In some machines the basket rotates independently of the tub and in other machines the basket and tub both rotate. In this invention, the rotatable structure is referred to generically as a “drum”, including the basket alone, or the basket and tub, or any other structure that holds and rotates the clothing load. Typically, an electric motor drives the drum. Various wash cycles introduce into the clothing and extract from the clothing the wash liquid, usually ending with one or more spin cycles where final rinse water is extracted from the clothes by spinning the drum.
It is common to categorize washing machines by the orientation of the basket. Vertical-axis washing machines have the basket situated to spin about a vertical axis relative to gravity. Horizontal-axis washing machines have the basket oriented to spin about an essentially horizontal axis, relative to gravity.
Both vertical and horizontal-axis washing machines extract water from clothes by spinning the drum about their respective axes, such that centrifugal force extracts water from the clothes. Spin speeds are typically high in order to extract the maximum amount of water from the clothes in the shortest possible time, thus saving time and energy. But when clothing and water are not evenly distributed about the axis of the drum, an imbalance condition occurs. Typical spin speeds in a vertical axis washer are 600-700 RPM, and in a horizontal axis washer at 1100 or 1200 RPM. Moreover, demand for greater load capacity fuels a demand for larger drums. Higher spin speeds coupled with larger capacity drums aggravates imbalance problems in washing machines, especially in horizontal axis washers. Imbalance conditions become harder to accurately detect and correct.
As the washing machine drum spins about its axis, there are generally two types of imbalances that it may exhibit: static imbalance and dynamic imbalance. FIGS. 1-4 illustrate schematically different configurations of imbalance in a horizontal axis washer comprising a drum 10 having a horizontal geometric axis 12. The drum 10 is suspended for rotation within a cabinet 14 having a front 16 (where access to the interior of the drum is normally provided) and a back 18. A drive point 19 (usually a motor shaft) is typically located at the back 18.
FIGS. 1(a) and (b) show a static imbalance condition generated by a static off-balance load. Imagine a load 20 on one side of the drum 10, but centered between the front 16 and the back 18. A net moment torque t causes the geometric axis 12 to rotate about the axis of rotation 22 of the combined mass of the drum 10 and the load 20 at an angular velocity ω, resulting in displacement d of the drum 10. This displacement, if minor, is often perceived as a vibration at higher speeds. The suspension system is designed to handle such vibration under normal conditions. Static imbalances are detectable at relatively slow speeds such as 85 or 90 RPM.
Dynamic imbalance is more complex and may occur independently of the existence of any static imbalance. FIGS. 2-4 illustrate several different conditions where dynamic imbalances exist. In FIGS. 2(a) and (b), imagine a dynamic off balance load of two identical masses 30, one on one side of the drum 10 near the front 16 and the other near the back 18. In other words, the masses 30 are on a line 32 skewed relative to the geometric axis 12. The net moment torque t1 about the geometric axis 12 is zero, so there is no static imbalance. However, there is a net moment torque t2 along the geometric axis 12, so that the drum will tend to wobble about some axis other than the geometric axis. If the moment is high enough, the wobble can be unacceptable.
FIGS. 3(a) and (b) illustrates a combined static and dynamic imbalance caused by a front off-balance load. Imagine a single load 40 in the drum 10 toward the front 16. There is a net moment torque t1 about the geometric axis 12, resulting in a static imbalance. There is also a moment torque t2 along the geometric axis 12, resulting in a dynamic imbalance. The resulting motion of the drum is a combination of displacement and wobble.
FIGS. 4(a) and (b) illustrates a combined static and dynamic imbalance caused by a back off-balance load. Imagine a single load 50 in the drum 10 toward the back 18. There is a net moment torque t1 about the geometric axis 12, resulting in a static imbalance. There is also a moment torque t2 along the geometric axis 12, resulting in a dynamic imbalance. The resulting motion of the drum is a combination of displacement and wobble.
Unfortunately, dynamic imbalance is often detectable only at higher speeds. Both vertical and horizontal axis machines exhibit static imbalances, but dynamic imbalances are a greater problem in horizontal-axis machines. Imbalance-caused vibrations result in greater power consumption by the drive motor, excessive noise, and decreased performance.
Many solutions have been advanced for detecting and correcting both static and dynamic imbalances. Correction is generally limited to aborting the spin, reducing the spin speed, or changing the loads in or on the basket. Detection presents the more difficult problem. It is known to detect vibration directly by employing switches, such as mercury or micro-switches, which are engaged when excessive vibrations are encountered. Activation of these switches is relayed to a controller for altering the operational state of the machine. It is also known to use electrical signals from load cells on the bearing mounts of the basket, which are sent to the controller. Other known methods sample speed variations during the spin cycle and relate it to power consumption. For example, it is known to have a controller send a PWM (Pulse Width Modulated) signal to the motor controller for the drum, and measure a feedback signal for RPM achieved at each revolution of the drum. Fluctuations in the PWM signal correspond to drum imbalance, at any given RPM. Yet other methods measure power or torque fluctuations by sensing current changes in the drive motor. Solutions for detecting static imbalances by measuring torque fluctuations in the motor abound. But there is no correlation between static imbalance conditions and dynamic imbalance conditions; applying a static imbalance algorithm to torque fluctuations will not accurately detect a dynamic imbalance. For example, an imbalance condition caused by a front off balance load (see FIG. 3) will be underestimated by existing systems for measuring static imbalances. Conversely, an imbalance condition caused by a back off balance load (see FIG. 4) will be overestimated by existing systems for measuring static imbalances.
Moreover, speed, torque and current in the motor can all fluctuate for reasons unrelated to drum imbalance. Commonly owned U.S. Pat. No. 6,640,372 presents a solution to factoring out conditions unrelated to drum imbalance by establishing a stepped speed profile where average motor current is measured at each step and an algorithm is applied to predetermined thresholds for ascertaining an unbalanced state of the drum. Corrective action by the controller will reduce spin speed to minimize vibration. The particular algorithm in the '372 patent may be accurate for ascertaining static imbalances. However, is not entirely accurate for horizontal axis washing machines because it does not accurately ascertain the various dynamic imbalance conditions.
There is yet another unacceptable condition of a rotating washer drum that involves neither a static or dynamic imbalance, but establishes a point distribution that can deform the drum. A point distribution condition is illustrated in FIG. 5(a) and (b). Imagine two identical loads 60 distributed evenly about the geometric axis 12, and on a line 52 normal to the geometric axis. There is no moment torque, either about the geometric axis 12, or along the geometric axis. Thus, there is no imbalance detectable at any speed. However, centrifugal force F acting on the loads 60 will tend to deform the drum. If the drum were a basket rotating inside a fixed tub as is common in many horizontal axis washers, the basket may deform sufficiently to touch the tub, degrading performance and causing unnecessary wear and noise.
Another problem in reliably detecting imbalances in production washers regardless of axis is presented by the fact that motors, controllers, and signal noise vary considerably from unit to unit. Thus, for example, a change in motor torque in one unit may be an accurate correlation to a given imbalance condition in that unit, but the same change in torque in another unit may not be an accurate correlation for the same imbalance condition. In fact, the problems of variance among units and signal noise are common to any appliance where power measurements are based on signals that are taken from electronic components and processed for further use.
There exists a need in the art for an imbalance detection system for a washing machine, particularly horizontal axis washing machines, which can effectively, efficiently, reliably and accurately sense any imbalance condition, and sense other obstructions that may adversely affect performance. Further, there is a need for accurately determining stable and robust power information that can accommodate variations in motors, controllers and signal noise from unit to unit.